1. Field of the Invention
The present invention relates to an image forming apparatus for photocopiers, printers, facsimiles, and so forth.
2. Description of the Related Art
A known color printer will now be described with reference to FIG. 18.
Recording paper 2 loaded on a cassette 1 is transported out by means of a pick-up roller 1a, with only the uppermost recording paper sheet being transported downstream by a feeding roller 1c and reverse-rotating separating roller 1b rotating in the direction opposite to the feeding direction. Next, the recording paper 2 is transported following the transporting path by means of a plurality of transporting roller pairs 1d, and the leading end thereof is retained by the nipping portion of a stationary resist roller pair 3 so that the recording paper 2 is bowed, thereby correcting any slant.
On the other hand, 6a, 6b, 6c, and 6d denote image forming means which form yellow, magenta, cyan, and black toner images on recording paper, respectively. Making description regarding 6a representatively, 6a1 denotes a photosensitive drum which rotates in the clockwise direction, 6a2 denotes a laser scanner which irradiates a laser beam upon the photosensitive drum 6a1 and forms an electrostatic latent image, and 6a3 is a developing unit which develops the electrostatic latent image formed on the surface of the photosensitive drum 6a1 using yellow toner.
The resist roller 3 starts rotating at a timing synchronous with the toner image formed on the photosensitive drum 6a1 and feeds the recording paper 2. The recording paper 2 which has been sent out is pressed against a charged transfer belt 5 by means of an adhesion roller 4, and the recording paper is subjected to electrostatic adhesion to the transfer belt 5.
The yellow toner image formed on the photosensitive drum 6a1 is transferred to the recording paper 2 which is transferred by the transfer belt 5, by transfer means 7a.
In the same way, magenta, cyan, and black toner images are transferred onto the recording paper 2 by means of the image forming means 6b, 6c, 6d, and transfer means 7b, 7c, and 7d.
8 denotes a heating fixing unit serving as fixing means, having a halogen heater built in a roller, which fixes onto the recording paper 2 the toner image transferred onto the recording paper 2 transported by the transfer belt 5.
The recording paper 2 which has passed through the fixing unit 8 is transported by an inner paper ejecting roller 9, and is ejected into a paper eject tray 11 by ejecting means comprising an outer paper ejecting roller 10.
Also, in the event that an image is to be formed on the opposite side of the recording paper 2 to which an image has already been formed on the first side thereof, a flapper 13 is rotated in a counter-clockwise direction, and the leading end of the recording paper 2 which has passed the inner paper ejecting roller 9 is guided to a reversal path 14 by means of the flapper 13. The recording paper 2 is transported to a switch-back path 31 by means of a first transporting roller 15 and a second transporting roller 16.
Once the trailing end of the recording paper 2 passes a reverse transport prevention guide 32 formed of a calcined sheet, the second transporting roller 16 is reversed so as to cause the recording paper 2 to switch-back. The leading edge of the recording paper 2 transported in the reverse direction is guided to a both-side transporting path 33 by means of the reverse transport prevention guide 32, and is transported by a both-side transporting roller 30 until reaching a transporting roller 1d.
The transporting roller 1d transports the recording paper 2 to the resist roller again, and an image is formed on the second side thereof by the same procedures as the process described above, thus completing image formation on both sides of the recording paper 2.
In the above image forming process, OHP (overhead projector) sheets or resin sheets, heavy paper (such as 157 g/m.sup.2 paper or 209 g/m.sup.2 paper), etc. is so thick that the paper absorbs the heat of the fixing unit, resulting in an imbalance with the thermal generating capabilities of the halogen heater, and consequently fixing is not performed correctly.
Particularly, with a color image forming apparatus such as illustrated in FIG. 18, a plurality of colors of toner are overlaid and transferred, so there is the necessity to increase the amount of heat more than normal, in order to cause the toner to completely melt and to develop transparency, since a greater amount of toner is deposited on the recording paper 2.
Raising the thermal capabilities of the halogen heater will deal with this problem, but creates other problems such as increased electrical current value, increased cost of the halogen heater, etc., so this problem is generally dealt with by lowering the transfer speed (the transporting speed of the recording paper) to increase the amount of time that the recording paper is passing through the fixing unit.
However, regarding the known example such as described above, the speed of the recording paper being ejected to the paper eject tray by the outer paper ejecting roller 10 also is decreased, causing problems such as those described below.
First, the base material for the OHP sheets is PET film, which softens due to the head of the fixing unit. As shown in FIG. 19, in the event that the softened OHP sheets are ejected with the outer paper ejecting roller 10, the leading edge thereof sags down to the paper eject tray 11 due to the weight thereof, and there has been the possibility of the OHP sheets curling up on the paper eject tray 11 as shown in FIG. 20.
In order to prevent this, ejecting needs to be completed before sagging under the sheet's own weight occurs. However, the output speed needs to be decreased for the aforementioned reason, and this has been a great problem regarding OHP sheet output.
FIG. 21 illustrates an arrangement in comparison with the known example, this arrangement being provided with a sorter 40 to the paper eject unit of a color printer 100 provided with sorting means for performing sorting of a plurality of copies of recording paper.
The recording paper 2 which has passed the outer paper ejecting roller 10 is ejected onto one of the sorter trays 42 of which a plurality are provided on the sorter 40, this ejecting being performed by means of a sorter roller 41. The sorter 40 has a plurality of sorter trays 42 arranged vertically. This sorting is performed by means of the plurality of sorter trays 42 being as a unit moved vertically, and the recording paper 2 which is being continuously output by the sorter roller being sequentially received on separate sorter trays 42.
According to the aforementioned known example shown in FIG. 18, the edge portion of the paper eject tray 11 to the side of the outer paper ejecting roller 10 is positioned immediately below the outer paper ejecting roller 10 (B in FIG. 18). On the other hand, the edge of the sorter tray 42 cannot be positioned immediately below the sorter roller 41, since the sorter trays 42 are to be moved vertically. Accordingly, in order to improve the loadability, the ejecting speed has to be set at a certain speed or faster in order to kick the recording paper 2 out, so that the trailing end of the recording paper 2 completely is separated from the sorter roller 41.
FIG. 22 denotes an arrangement wherein a finisher 50 is attached to the paper eject unit of a color printer 100. The finisher 50 comprises folding means for folding the ejected recording paper and binding means for stacking a plurality of recording sheets and binding these (i.e., bookbinding means).
Now, description will be made regarding the binding means. The recording paper 2 transported by the outer paper ejecting roller 10 is guided to a first roller 51 by means of a flapper 51, and is ejected into a stack tray 54 by means of a second roller 53. The recording paper 2 is transported until it is pressed against a truing restricting plate 54a by means of unshown transporting means, each time a sheet is ejected onto the stack tray 54, thus forming a trued sheet stack. Once a trued sheet stack of one group of recording paper sheets has been formed, the stack is bound with staples 55, and is transported in the direction shown by the arrow E and is ejected on a first finisher tray 56.
Next, description will be given regarding the folding means. In the event that the flapper 51 is at the position shown by the broken line, the recording paper 2 transported by the outer paper ejecting roller 10 is guided to a third roller 57 and transported to a transporting path 58.
When the leading edge of the recording paper 2 abuts a stopper 58a of the transporting path 58, a jutting plate 59 moves in the direction shown by an arrow and presses the center portion of the recording paper 2 in the direction of transport. The recording paper 2 is bent into two from the center portion in the transporting direction as can be seen from the recording paper 2a which is nipped by the nipping portion of the center folding roller 60, and output on a second finisher tray 61.
Also, a plurality of sheets of recording paper 2 may be stacked in the transport path 58 and bent in two by the center folding roller 60, even in a stacked state.
Further, a plurality of sheets of recording paper 2a can be bound at the center folding portion by means of unshown stapling means, thus forming a set of recording paper sheets.
With the known example, the post-paper-eject processing devices such as the sorter 40 or the finisher 50 need to correspond with the decreased transporting speed of the recording paper until the recording paper 2 passes through the outer paper ejecting roller 10.
Accordingly, driving switching means for dealing with the slow transporting speed are required for the post-paper-eject processing devices, resulting in increased costs.
Also, as described above, with the sorter 40, the ejecting speed has to be set at a certain speed or faster in order to kick the recording paper 2 out, so that the trailing end of the recording paper 2 completely is separated from the sorter roller 41, and thus there is the need to increase the transporting speed once the recording paper 2 has passed through the fixing unit 8.
For example, in order to change the number of rotations of the motor as the aforementioned driving switching means, there is the need to change the number of rotations of the motor to a high rotation speed within the time from the trailing edge of the recording paper 2 passing the outer paper ejecting roller 10 to the trailing edge of the recording paper 2 passing the sorter roller 41, and in the event that the acceleration of the motor is not sufficient the distance between the outer paper ejecting roller 10 and the sorter roller 41 must be extended, this increasing the size of the apparatus.
Also, the post-paper-eject processing devices are constructed separately from the image forming apparatus, and are arranged so that the transporting speed thereof can be changed within a certain range, in order to allow common connection of the post-paper-eject processing devices to a plurality of types of image forming apparatuses.
For example, in order to allow connection of the post-paper-eject processing devices to image forming apparatuses with a transporting speed of 100 mm/s to image forming apparatuses with a transporting speed of 400 mm/s, the post-paper-eject processing devices are also provided with a driving system which is capable of changing the transporting speed from 100 mm/s to up to four times, 400 mm/s.
However, in the event that the transporting speed for OHP sheets or heavy paper is 25 mm/s, there is the need to accelerate the transporting speed of the post-paper-eject processing devices from 25 mm/s to 400 mm/s, which is acceleration of 16 times, and the driving motor which can be used for such is limited.